1. Field of the Invention
The invention relates to an amplifier stage of constant input impedance. The amplifier stage of the invention is a regulatable amplifier stage.
2. Description of the Related Art
An emitter-coupled amplifier stage for differential amplification is known from Electronic Circuits: Design and Applications, U. Tietze and Ch. Schenk, Springer-Verlag, Berlin 1991, pp. 60-66. A basic circuit of that differential amplifier is illustrated in FIG. 1. A first transistor T1 and a second transistor T2 are connected to one another via the emitter terminals. The common emitter lead is connected to a first reference potential M via a current source I through which a current I.sub.0 flows. Input voltages UE1 and UE2 are applied to input terminals E1 and E2, which are connected to the base terminals of the respective transistors T1 and T2. The collectors of the transistors T1 and T2 are connected to output terminals A1 and A2, at which the amplified differential voltage can be picked up. The amplification is adjusted by means of the current source I. The primary disadvantage of this circuit is that the input impedance of the two differential amplifier branches varies sharply with quiescent current.
A differential amplifier is a symmetrical direct voltage amplifier with two inputs and two outputs. The current source in the common emitter lead is definitive. It has the effect that the sum of the emitter currents IE1+IE2=I.sub.0 remains constant. In the quiescent state, the input voltage at the terminal El is equal to the input voltage at the terminal E2, which in turn is zero (UE1=UE2=0). In that case, the constant current I.sub.0 for reasons of symmetry is distributed uniformly to the two transistors T1 and T2. The differential amplification (differential-mode gain) A.sub.D becomes ##EQU1## and the differential voltage U.sub.D becomes U.sub.D =UE1-UE2.
However, if the input voltages are varied so that UE1 for instance is greater than UE2, then the current distribution in the differential amplifier changes, so that IC1 (the collector current in the transistor T1) increases and IC2 (the collector current in the transistor 2) decreases. However, the sum of the currents IC1+IC2 remains constant. A difference in the input voltage accordingly causes a change in the output voltage. Differential amplifiers are also used whenever it is not a voltage difference but rather only one of the two input voltages, UE1 or UE2, that is to be amplified. In that case, one of the two inputs E1 or E2 is connected to zero potential.
A special differential amplifier stage has been disclosed by German published, prosecuted patent disclosure DE 28 04 064. There, an emitter-coupled differential amplifier stage is connected to a base-coupled differential amplifier stage. The collectors of the two stages are cross-coupled and form the outputs of the amplifier circuit.